The present invention relates to cellular and other communication systems, and more particularly, to methods and apparatus for achieving additional data rates in communication systems utilizing code division multiple access (CDMA).
When multiple users share a transmission medium, some form of multiplexing is required to provide separable user sub-channels. There are many multiplexing techniques available which simultaneously transmit information signals within the available bandwidth, while still maintaining the quality and intelligibility that are required for a given application. Code division multiple access (CDMA) techniques, for example, transmit multiple information signals on the same channel, and differentiate each user sub-channel by encoding it with a unique spreading code.
Code division multiple access (CDMA) techniques have been employed in many digital wireless communication systems to permit a large number of system users to communicate with one another. Current CDMA networks were designed to carry only voice traffic and exhibit limited data rate variability. CDMA networks, however, must evolve to encompass a variety of multimedia applications, each having potentially different data rates. Thus, CDMA networks will be required to carry information associated with the various multimedia applications at various data rates, corresponding to the requirements of diverse wireless services demanded by customers.
Many communication systems utilizing code division multiple access (CDMA) comply with the IS-95 standard, adopted by the Telecommunication Industry Association (TIA). Under the IS-95 standard, a communication system substantially eliminates co-channel interference and improves the bit energy-to-noise density ratio, Eb/No, on the forward link from a base station or cell site to a mobile receiver unit by modulating the information signals with Walsh orthogonal function sequences. To produce corresponding orthogonal information signals, these CDMA systems require that the forward link information signals be transmitted in a synchronized manner.
A more detailed discussion of the IS-95 standard is provided in xe2x80x9cMobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System,xe2x80x9d Telecommunication Industry Association Doc. No. TIA/EIA/IS-95 (1993)[, incorporated by reference herein]. The document TIA/EIA/IS-95 (1993) may be obtained, for example, using the following Internet reference: http://www.tiaonline.org/standards/search_n_order.
Current implementations of the IS-95 standard, however, allow only a limited number of data rates. Specifically, the Telecommunication Industry Association has recently adopted a new standard, IS-95B, for increasing the data rates that may be achieved with CDMA networks. The IS-95B standard, however, only permits data rates that are integer or power-of-two multiples of the basic CDMA rate. While such techniques satisfy the data rate needs of many applications, the utility of CDMA networks could be significantly extended if further data rate variability could be achieved.
Generally, a CDMA communication system offering a near continuum of data rates up to a maximum rate is disclosed. The disclosed CDMA communication system increases the data rate variability, without disturbing the orthogonality between users. An increased number of data rates is obtained by time multiplexing the data rates achievable with conventional CDMA systems to provide additional data rates for various multimedia applications.
In accordance with one feature of the present invention, if a user is assigned a specific orthogonal code sequence, such as a Walsh sequence, which permits a nominal data rate, R, and higher data rates, nR, the user can obtain additional desired data rates by time multiplexing the specific orthogonal code set. In this manner, desired bit rates are provided for applications requiring bit rates that are not an integer multiple of the rates provided by conventional xe2x80x9cfat pipexe2x80x9d techniques.
In an illustrative sixteen dimension implementation, if a specific code quadruple, such as the code quadruple (wk, wk+4, wk+8, wk+12) (k=0, 1, 2, 3), is assigned to an information source, any rate can be achieved, up to a rate of 4R. Rates R, 2R, 4R and 16R can be achieved by assigning other appropriate code sets to the information source, in a conventional manner. According to a feature of the invention, any desired rate up to 4R is achieved by time multiplexing the codes wk, wk2 and wk4. The symbol, wkn, denotes the first nth of the spread spectrum code wk (hence, n=2 denotes the first half of the sequence). Thus, the user can achieve any data rate that is a rational scale factor combination of the rates R and 4R.
The transmitter of the present invention achieves a rate conversion to match the appropriate codeword to the appropriate data rate, by employing buffering and reclocking of the data, with time multiplexing of the orthogonal Walsh codes corresponding to the available lower and upper rates. The rate conversion must be synchronized with the selection of the orthogonal Walsh code (corresponding to the lower and upper rates). For example, an application requiring a data rate between 2R and 4R, such as 2.3R, would require multiplexing of the 2R and 4R Walsh codes (wk2 and wk4) to achieve the desired 2.3R rate. If the 2.3R application was assigned the specific code quadruple (w1, w5, w9, w13) the sequence w12 is utilized to achieve a rate of 2R and the sequence w14 is utilized to achieve a rate of 4R, with an appropriate balance to achieve an overall 2.3R rate, and the codes w5, w9, w13 are excluded from use.